; PDP-11 CRC-32 code

; r0=byte
; r1=>data
; r2=count
; r3/r4=crc, r4=lo, r3=hi
; r5=polynomic

.bytelp
movb (r1)+,r0	; Fetch byte from memory
bic #&FF00,r0	; Ensure b8-b15 clear

; The following code updates the CRC with the byte in R0
xor r0,r4	; XOR into CRC low byte
mov #8,r0	; Prepare to rotate 8 bits
.rotlp
clc
ror r3		; Rotate CRC
ror r4
bcc clear	; Carry out clear
mov #&EDB8,r5	; CRC=CRC xor &EDB88320
xor r5,r3
mov #&8320,r5
xor r5,r4
.clear
sub #1,r0
bne rotlp	; Loop for 8 bits

sub #1,r2	; num=num-1
bne bytelp	; Loop until num=0
mov r3,r1	; Return CRC to BASIC in R0/R1
mov r4,r0
rts pc		; Return with R3/R4=updated CRC